Distributed Optical Fibre Sensing

ABSTRACT

There is disclosed a distributed optical fibre sensing system in which the sensor fibre comprises at least first and second waveguides used for separate sensing operations. The sensor fibre may be, for example, a double clad fibre having a monomade core and a multimode inner cladding.

The present invention relates to apparatus and methods for distributedoptical sensing, in which parameters such as temperature, strain andvibration are measured along extended lengths of a sensor optical fibre,as a function of position along the sensor fibre, by detectingproperties of light scattered within the fibre.

INTRODUCTION

Distributed optical fibre sensing is a well known approach to providinginformation about environmental conditions along the length of a sensoroptical fibre which can extend for considerable distances alongstructures such as pipelines, cables, building and bridges, downboreholes, and in numerous other applications. In principle, spatiallyresolved information about environmental conditions can be obtained fromevery point along the sensor fibre, subject to issues such as spatialresolution, and reduced signal strength as the range is extended.Environmental parameters that can be sensed through their correspondinginfluence on the optical fibre include temperature, static strain andvibration, and various optical techniques have been used separately andin combinations to make improved measurements of one or more suchvariables. Other parameters such as pressure can also be sensed, forexample by suitable packaging of the optical fibre to convert a pressureeffect into a local strain effect.

Distributed optical fibre sensing involved launching probe light intothe sensor fibre, and detecting properties of probe light received at aninterrogator, usually via a back-scattering process. The differenttechniques which may be used to analyses the scattered light includeanalysis of Rayliegh scattering which involves no frequency shift of theprobe light, analysis of the Raman scattered spectrum, and analysis ofthe Brillouin scattered spectrum.

Rayleigh scattering is a relatively strong effect caused by theinhomogeneity of the fibre. It may be accompanied by scattering offdiscontinuities such as point imperfections and joints in the fibre, orfrom more gradual sources of attenuation such as bends and changes inrefractive index. To establish the location of discontinuities to withindistances of the order of a metre, very short pulses of probe light, forexample just a few nanoseconds may be used. More gradual changes inrefractive index may be caused by physical strain and temperaturechanges, and can be detected by variations in the phase of the Rayleighscattered light using much more coherent, longer pulses of probe light.The phase variations may be detected as a temporal “speckle pattern” atthe interrogator as slight timing differences in travel velocity alongthe length of the fibre due to refractive index variations causereturning scattered light to self-interfere. Such coherent Rayleighbackscatter techniques are discussed, for example, in WO2008/056143.Rayleigh scattering techniques, generally referred to as optical timedomain reflectometry (OTDR) can be used to detect changes which arefairly rapid over time, for example at acoustic frequencies, because thehigh scattering cross sections lead to relatively short detectorintegration times, as well as very location specific defects such asfibre damage.

Brillouin scattering occurs when probe light scatters off phonons in theoptical fibre. Probe light is scattered to both higher and lowerwavelengths, typically by about 10 GHz, due to the Doppler shift effectof phonons moving towards and away from the interrogator. The magnitudeof the frequency shift and the relative intensities of the usual twopeaks depend upon a combination of fibre temperature and strain, andcareful examination of the Brillouin spectrum alone can be used todetect the separate parameters, as described in Parker et al. IEEEPhotonics Technology Letters, vol 9(7) July 1997, p 979-981. The totalBrillouin scattering cross sections are generally about one to twoorders of magnitude weaker than Rayliegh scattering.

Raman scattering occurs when probe light interacts with molecular bondsin the optical fibre. As for Brillouin scattering, probe light isscattered to both higher and lower wavelengths. However a broaderspectrum having a detailed structure characteristic of the chemistry ofthe scattering medium is generated, with particular spectral featuresaround 13 THz from the probe frequency being used. Although the totalRaman scattering cross section might typically be only two orders ofmagnitude weaker than the total Brillouin cross section, the intensityof individual Raman spectral features will be far weaker still. TheRaman spectrum is dependent on temperature, but not on strain, so may beused in combination with a Brillouin technique to derive a lessambiguous measure of strain or vibration in the optical fibre.

It is also known to use two or more separate sensing techniques at thesame time in order to take advantage of the different characteristics ofeach. WO2007/104915 describes the simultaneous use of Brillouinbackscatter with coherent or broadband Rayleigh noise to detect rapidlyand more slowly varying characteristics of a structure such as apipeline.

Interrogation techniques which rely on Rayleigh and Brillouin scatteringare best performed using a single mode optical fibre, to reduce modeldispersion. Interrogation techniques which rely on Raman scatteringgenerally require much higher probe light intensities which can causedamage, especially at connectors and joints, or undesirable effects suchas stimulated Brillouin scattering in the narrow core of a single modefibre, so are best performed using multimode optical fibre. Suitablemultimode fibres need not be any larger in overall diameter than asingle mode fibre, but the guiding region is normally much larger thanthat inside a single mode fibre, and the refractive index differencebetween the core and the cladding is also usually much larger. Multimodefibre designs vary widely. Optical guidance can be accomplished by usinga uniform glass core surrounded by a polymer with low refractive index,by doping an outer glass region to lower its refractive index, or bydoping a central region of the fibre to increase its refractive index.Depending on the dopant type and concentration, uniform or gradedrefractive index distributions can be created, and all of these designsare in principle compatible with distributed fibre sensing use Ramanscattering.

It would be desirable to address these and other problems of the relatedprior art.

SUMMARY OF THE INVENTION

The invention provides a distributed optical fibre sensor system inwhich multiple waveguides of a single optical fibre are used toimplement more than one sensing operation or sensing technique usingbackscatter of probe light within the same sensor fibre. In particular adouble clad fibre, which typically combines a single mode waveguide witha multimode waveguide typically provided by an inner cladding, may beused so that sensing techniques which benefit from use of a multimodefibre, such as those using Raman scattering, can be implemented inparallel with techniques which benefit from use of a single mode fibre,such as those using Rayleigh or Brillouin scattering, without needing touse multiple parallel sensor fibres. The invention therefore provides adistributed fibre sensor apparatus in which the sensor fibre is, orcomprises at least a length of a double clad fibre. Of course, thesystem can be arranged to use various combinations of multiple sensingoperations and techniques with the multiple parallel waveguides of thesensing fibre, and systems may be envisaged in which sections of variousdifferent fibre types are used and interlinked.

The use of a double clad fibre as the sensor fibre also enables pumplight to be delivered along the multimode waveguide to a distant opticalamplifier section of the sensor fibre, where signal amplification isdesired, for example to amplify the probe light used to carry out asensing operation. This can be used to improve the range and/orsensitivity of a sensing operation, especially a sensing operation usingthe single mode waveguide, whether or not a parallel sensing operationis also using the multimode waveguide. The invention therefore alsoprovides a distributed fibre sensor apparatus in which the sensor fibrecomprises a double clad fibre and an optical amplifier section, whichmay also be of double clad fibre. An additional pump light source isalso required, for connection to the multimode waveguide of the sensorfibre so as to drive the optical amplifier.

This aspect of the invention may be helpful to compensate for a doubleclad fibre having higher attenuation than an alternative single corefibre, or for significant attenuation in connectors.

If high probe light powers are required for a sensing operation using asingle mode waveguide, then such high powers can cause damage atconnectors, especially connectors closer to the probe light source. Ifprobe light in the single mode waveguide is amplified to required powerlevels after particular connector interfaces have been traversed, thenthe conditions for optically induced damage at the connectors can beavoided, because equivalently high pump powers are spread out over therelatively large area of the multimode waveguide, reducing the opticalintensity at the connectors below any optical damage threshold.

In the prior art, double clad fibre is frequently used to facilitate theconstruction of active optical devices such as optical fibre lasers andoptical amplifiers, in which the single mode waveguide carries “signal”light having a first wavelength, and in which the single mode waveguideis doped with an active optical dopant which absorbs “pump” lightcarried in the multimode waveguide. The optical amplifier section of thepresent invention may comprise a section of such doped double cladfibre, while other sections of the sensor fibre may be provided byundoped, or passive sections of double clad fibre. The sensor fibre, orsections such as passive sections of the sensor fibre as appropriate,may be selected or designed to provide the lowest practical attenuationin order to extend the useful range of the sensor. The sensor fibre, orsections of the fibre may also be designed to provide other desirablecharacteristics, such as particular dispersion or scattering properties,in order to further optimise the range or sensing performance or both.

Double clad fibre used for the sensor fibre may include joints andconnectors as necessary or practical. Sections of other fibre types mayalso be used in the sensor fibre, for example a final length of singlemode fibre distant from an interrogator function, or linking sections ofseparate single and multi mode fibres.

Another aspect of the invention is to use a pump-signal combinercomponent to make the single mode and multimode waveguides of the doubleclad sensor fibre available for sensing operation simultaneously, bylinking optical fibres from interrogation functions or units into theseparate waveguides of the sensor fibre with low connection losses andwithout blocking either path.

Although double clad fibres and pump signal combiners are usuallydesigned to minimise cross-talk between the multimode and single modewaveguides, they may not be perfect in this regard, and such cross-talkcould be detrimental to the independent operation of interrogatorssimultaneously connected to the sensor fibre for sensing operations.Such cross-talk may therefore be minimized by using probe light ofdifferent wavelengths between different interrogators and/or waveguides,and using optical filters to increase isolation between interrogatorssharing the double clad fibre.

Separation between launched and backscattered probe light is typicallyaccomplished in distributed fibre sensors, as well as other instruments,by using passive components such as splitters and circulators. In adistributed sensing system using double clad fibre, these componentsshould be selected to be compatible with the fibre types used, and maybe separately integrated into the interrogator optics for the singlemode and multiple mode sensing operations, on the instrumentation sideof the pump-signal combiner.

Typically, the sensor fibre may extend at least a hundred metres fromthe interrogation system and frequently tens of kilometres. When theabove mentioned optical amplifier is implemented in the sensor fibre, itmay typically be used at least a hundred metres from the interrogationsystem, although it may also be useful to implement the opticalamplifier much closer to the interrogation system for example less thana metre along the sensor fibre, if the intended purpose of the amplifieris to avoid high probe light powers damaging connectors proximate to orforming part of the interrogation system.

Accordingly, the invention provides a distributed optical fibre sensingsystem, for example for monitoring an environment, comprising: a sensorfibre, extending through the environment, having first and secondwaveguides; and an interrogation system coupled to the sensor fibre andarranged to use probe light in the first waveguide to carry out at leasta first sensing operation to obtain scattering information from multiplelocations, or distributed along the sensor fibre and to use other probelight in the second waveguide to carry out at least a second sensingoperation to obtain other, different scattering information frommultiple locations, or distributed along the sensor fibre. Thescattering information is then used to derive one or more physicalparameters relating to the sensing fibre and/or its environment (such astemperature, strain, vibration, bending, pressure, etc). In particular,scattering information from two or more sensing operations, and inparticular form the two waveguides, may be combined to carry out such aderivation.

The first and second waveguides may be adjacent, c-axial, or otherwiseco-extending along the sensor fibre.

The sensor fibre may be constructed such that the first waveguide is asingle mode waveguide and the second waveguide is a multimode waveguide.In particular, the first and second adjacent waveguides may bewaveguides of a double clad optical fibre, in which the single modewaveguide is a single mode core of the double clad fibre. The firstsensing operation may utilize at least Brillouin backscatter or Rayleighscattering within the single mode waveguide to obtain scatteringinformation from along the sensor fibre, and the second sensingoperation may utilize Raman scattering within the multimode waveguide toobtain different scattering information from along the sensor fibre.

More generally, however, the sensing operations may each use one or moreof a variety of techniques including Rayleigh scattering, optical timedomain reflectometry, coherent or partially coherent optical time domainreflectometry, spontaneous or stimulated Brillouin scattering, andspontaneous or stimulated Raman scattering, noting that these terms arenot all intended to be mutually exclusive.

The interrogation system may be arranged to carry out the first andsecond sensing operations utilizing light of different wavelengths tothereby avoid interference between the first and second sensingoperations arising from leakage between the first and second waveguide.The interrogation system may also be arranged to carry out multiplesensing operations using one of said first and second waveguidesutilizing probe light of different wavelength for each said sensingoperation in the one waveguide.

The system may further comprise a pump light source coupled to thesecond waveguide. The sensor fibre may then include a section remotefrom the interrogation system configured as an optical amplifier suchthat probe light in the first waveguide is amplified by the opticalamplifier.

The system may comprise a pump-signal combiner arranged to opticallycouple the interrogation system to the first and second waveguide. Adouble-clad fibre section of the sensor fibre may then be terminated atthe pump-signal combiner, so that the pump-signal combiner couples thefirst and second waveguides of the double clad fibre separately to atleast first and second interrogation units of the interrogator system.For example, the first interrogation unit may be coupled by thepump-signal combiner to the first, single mode waveguide of the doubleclad fibre so as to determine Rayleigh and/or Brillouin backscatterproperties of the sensor fibre, and the second interrogation unit may becoupled by the pump-signal combiner to the second, multi mode waveguideof the double clad fibre so as to determine Raman backscatter propertiesof the sensor fibre.

The invention also provides a distributed optical fibre sensing systemcomprising: a sensor fibre; a pump light source arranged to deliver pumplight into the sensor fibre; and an interrogator system coupled to thesensor fibre to obtain scattering information from along the sensorfibre, the sensor fibre comprising a first section proximal to theinterrogator, a second section distal from the interrogator, and anoptical amplifier section between the first and second sections, atleast the first section of the sensor fibre comprising double cladoptical fibre having a single mode waveguide and a multimode waveguide,the interrogator system being coupled to the single mode waveguide todetect scattering information from along the sensor fibre using probelight injected into the single mode waveguide, and the pump light sourcebeing coupled to the multimode waveguide, the system being arranged suchthat the optical amplifier section amplifies said probe light using saidpump light.

In particular, the optical amplifier may comprise an optically activelength of double clad fibre.

The interrogator system may also be coupled to the multimode waveguideof the first section of the sensor fibre, to also detect scatteringinformation from along the sensor fibre using probe light injected intothe multimode waveguide. The interrogator systems may then use a firstsensing technique to detect scattering properties of one or more of thesingle mode waveguide sections of the sensing fibre, and a secondsensing technique to detect scattering properties of one or more of themultimode waveguide sections of the sensor fibre.

At least a part of the second section of the sensor fibre may comprise asingle clad fibre coupled to the single mode core of the first sectionof the sensor fibre through the single mode core of the opticalamplifier section, such that the interrogator system can determinescattering properties along the single clad fibre using said probe lightinjected into said single mode waveguide.

The interrogator system may comprise one or more interrogator units eacharranged to carry out a separate sensing operation to determinescattering properties along the sensing fibre. The system may be furtheradapted to derive one or more physical properties along the sensingfibre from said scattering properties.

In accordance with the above discussion, the invention generallyprovides a distributed fibre sensing system arranged to detectscattering properties along a sensor fibre, the sensor fibre comprisingat least a length of double clad optional fibre.

The invention also provides methods corresponding to the above systemsand apparatus. For example, the invention provides a method of carryingout distributed fibre optic sensing by using two waveguides of a singlefibre, such as the single mode and multimode waveguides of a double cladfibre, to carry out two or more different sensing operations. Morespecifically, the invention may provide a method of determining physicalparameters along an extended path through an environment, comprising:running a sensor optical fibre along the path through the environment,the sensor fibre having first and second waveguides as discussed above;performing separate sensing operations using the first and secondwaveguides, to determine scattering properties of probe light atmultiple locations along the extended path; and determining saidphysical parameters from said scattering properties.

The invention also provides methods, and corresponding apparatus, fordistributed vibration sensing using Rayleigh backscatter within amultiple mode optical fibre, comprising coupling an interrogator to themultiple mode optical fibre using a double clad optical fibre having asingle mode waveguide and a multiple mode waveguide, receiving at theinterrogator, through the single mode waveguide of the double cladoptical fibre, light backscattered within the multiple mode opticalfibre, and detecting vibrations as a function of position along themultiple mode optical fibre from the received backscattered light. Thistechnique may be used, for example, to implement coherent or partlycoherent Rayleigh backscatter detection of vibrations in a well bore inwhich a multiple mode fibre has already been installed, and allowscontinued use of the installed multiple mode fibre for other purposessuch as distributed temperature sensing within the well bore.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a distributed optical fibre sensor according to theinvention, including two interrogators coupled to separate waveguides ofa sensor fibre 10;

FIG. 2 is a cross section through a double clad optical fibre which maybe used as the sensor fibre 10 of FIG. 1;

FIG. 3 illustrates variations on the distributed optical fibre sensor ofFIG. 1, using four interrogator units instead of two;

FIG. 4 illustrates a further variation of a distributed optical fibresensor according to the invention, in which a pump source 60 is used todrive an optical amplifier section 54 in the sensor fibre 50;

FIG. 5 illustrates a variation to FIG. 4, in which two interrogators 14are coupled to separate waveguides of the sensor fibre 50;

FIG. 6 shows an application of the invention in a well bore; and

FIG. 7 shows an alternative use of double clad optical fibre to couplean interrogator to a multiple mode optical fibre already installed in awell bore or other environment.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to FIG. 1 there is shown an example distributed opticalsensing system according to the invention. A sensor fibre 10, which maybe housed for example as part of a cable structure, extends through anenvironment to be monitored, for example along a structure 12 such as apipeline. The sensor fibre is a double clad optical fibre, althoughother fibre types having two or more waveguides extending together alongthe fibre may be used. The sensor fibre 10 is coupled to an interrogatorsystem 11. In FIG. 1 the interrogator system comprises two interrogatorunits 14′ and 14″, which are both coupled to the sensor fibre using acombiner 13. Each interrogator unit 14 includes a probe light source 16,an optical analyser 18, and a couple 20 to link both the respectiveprobe light source 16 and the respective optical analyser 18 to a fibrerunning to the combiner 13. Typically, each probe light source 16 mightinclude a laser such as a semiconductor laser, driving electronics forcontrolling the laser, temperature control for the laser, opticalfilters to condition the probe light before delivery to the combiner,and so forth. Typically, each optical analyser 18 might include opticalamplifier and filter elements, along with spectrometer and/orphotodetector capability, which can be operated to carry out thespectral and/or temporal analysis of the backscattered probe light toderive the scattering information necessary to determine the relevantenvironmental and/or physical parameters such as temperature, strain andvibration at various points aloud the sensor fibre. Each coupler 20could be provided by a 50/50 optical fibre coupler or optical circulatoror similar.

The double clad optical fibre provides at least two waveguides. Thecombiner 13 operates to couple one of the interrogator units 14′ to afirst of the waveguides, and the second interrogator unit to a second ofthe waveguides. As illustrated in section in FIG. 2, a typical doubleclad fibre comprises a relatively narrow single mode core waveguide 30,surrounded by a larger inner cladding 32 which forms a multi-modewaveguide. The inner cladding is surrounded by an outer cladding 34. Forconvenience of fabrication and joining to other fibres, the single modecore may be concentric within the inner and outer cladding layers, butthis need not be the case, and non-concentric and non-circularwaveguides may be used. Similarly, a circular single mode core may beadvantageous and convenient, but other core section shapes may be used.

Generally, double clad optical fibres are constructed using steps inrefractive index between the different parts to form the waveguides, butgraduated refractive index changes may also be used. Constructiontechniques may include using different materials for the different partsof the fibre including glass and polymer materials, and the doping ofmaterials to vary the refractive index of parts of the fibre.

The coupler 13 may be implemented using a known “pump-signal combiner”component, which is a passive optical component used in the prior art tocouple a pump beam and a signal beam into a double clad fibre laseramplifier. The pump signal combiner provides access to both waveguidesof the double clad fibre simultaneously, with low optical losses.

Data acquired by each optical analyser 18 is processed, and if necessarydisplayed and/or stored by a data processor 22, which could be a generalpurpose personal computer, or a more specifically constructed dataprocessing system. For example, the data processor may receive spectralor temporal scattering information in a raw, processed, parameterised orother form, from the optical analysers and process this information toderive relevant physical and other parameters such as temperature andstrain values and/or profiles along sections of the sensor fibre fordisplay, storage, or delivery over the network. Scattering data from twoor more sensing operations may be combined to service particularparameters. For example, scattering data from a Brillouin sensingoperation and a Raman sensing operation may be combined to derive animproved measure of strain along the sensing fibre. Scattering data andphysical parameters may be gathered, processed and derived in respect ofparticular points, particular sections or the entire length of thesensor fibre, as desired or required by a particular application.

The optical analysers 18′, 18″ and data processor 22 may be combinedinto a single unit for convenience of use and installation, and may beconnected to a wider data network 24 such as the Internet for remoteaccess to the data. Separate data processing capability may be providedfor each optical analyser.

In use, each light source 16 delivers carefully controlled probe lightinto the sensor fibre 10, through each coupler 20′, 20″ and the combiner13. The probe light interacts in various ways with the sensor fibre 10,and some of the scattered light returns back to each interrogator whereit is analysed. Various properties of the scattered light can bemeasured to determine properties of the optical fibre 10 and thereforeof its environment for example the structure 12. The data processor 22may be used to derive, and if required display, store or deliver overthe network particular physical parameters or profiles of parameters,such as temperature, static and dynamic strain along some or all of thedouble clad fibre 12.

Preferably, a first of the optical analysers 14′ is adapted to carry outRayleigh scattering and/or Brillouin scattering analysis of thescattered light, and is coupled to the single mode core waveguide of thedouble clad optical fibre. The second analyser 14″ is adapted to carryout Raman scattering analysis of the scattered light, and is coupled tothe multi-mode inner cladding waveguide of the double clad opticalfibre. In this way two or more different analysis techniques may beoptimised without need to use two or more separate sensor fibres 10.

Although the double clad fibre may be designed to minimise cross-talkbetween the waveguides and the combiner, this will rarely be perfect andsome probe light from each interrogator unit will therefore berepresented in the scattered light received by the other. The lightsources 18′, 18″ may therefore be arranged to deliver probe light ofdifferent wavelengths to the different waveguides, to help suppressinterference between the interrogator units, and improve the results ofthe scattering analysis carried out by each interrogator unit. To thisend, optical filters tuned to the different waveguides may be used, forexample as part of each interrogator unit, to increase the opticalisolation between the two interrogator units.

It will be apparent to the skilled person that more than twointerrogator units may be used, analysing the scattered probe lightusing one, two, or more than two different types of analysis, forexample selected from Rayleigh scatter methods such as OTDR and coherentOTDR, as well as Brillouin backscatter and Raman scattering techniques.A variation of the distributed fibre sensing system of FIG. 1 is shownin FIG. 3. In this system, for optical backscatter interrogator units*14 a . . . , 14 d) are shown connected to the sensing fibre 10, withtwo “single mode” interrogator units 14 a, 14 b being connected to thesingle mode waveguide and two “multi mode” interrogator units 14 c, 14 dconnected to the multlimode waveguide. The two single mode interrogatorunits 14 a, 14 b are coupled using a wavelengthmultiplexer/demultiplexer 40 enabling each of the two single modeinterrogator units to operate using the single mode waveguide using adifferent wavelength or waveband of probe light. The two multi-modeinterrogator units 14 c, 14 d are similarly coupled using a wavelengthmultiplexer/demultiplexer 42.

In FIG. 3 details of the interrogator units and their connectivity todata processor functions are not shown, but may be similar to thoseshown in FIG. 1. The interrogator units of FIG. 3 may be used to applyfour analysis techniques simultaneously. For example, interrogator unit14 a may operate using coherent Rayleigh noise to detect vibration, andinterrogator unit 14 b may operate using Brillouin scattering to measurestatic strain, both using the multimode waveguide of the sensor fibre10. At the same time, interrogator unit 14 c may operate using Ramanscattering to measure temperature, and interrogator unit 14 d mayoperate to measure fibre attenuation, for calibration or other purposes,both using the single mode waveguide of the sensor fibre 10.

Although the different analysis techniques may be carried out usingseparate interrogator units, it is possible to combine parts of all ofthe different interrogator units so that separate interrogator functionsare implemented using common elements. For example, two or more separateinterrogator functions may share a common laser, spectometer or othercomponents, or may be implemented entirely using common opticalcomponents.

Another embodiment of the invention is illustrated in FIG. 4. In thisembodiment the sensing fibre 50 includes at least three sections. Afirst section 52 is made of a double clad fibre having a single modewaveguides and a multi mode waveguide. A second section 56 may be madeof double clad or more conventional single clad fibre. Between the firstand second sections is an optical amplifier section 54, formed of alength of double clad fibre suitable doped to make it optically active.The first and second sections and the amplifier section may be joinedusing conventional fibre connectors and/or splice joints as appropriate,and may include other sections, joints and connectors as required.

The first section 52 of the sensor fibre is also connected to a pumpsource 60 and an interrogator unit 14′. The interrogator unit 14′ may besimilar to those already described, and is coupled to the single modecore of the first section of the sensor fibre through coupler 13. Theinterrogator delivers probe light into the sensor fibre, and collectsprobe light scattered back from the sensor fibre, using one or moreanalysis techniques to detect properties distributed along one or moresections of the sensor fibre.

The pump source delivers pump light into the multi mode waveguide of thefirst section of the sensor fibre. The pump light is delivered throughthe first section of sensor fibre to the amplifier section where it isused to amplify the probe light, thereby increasing the strength of thesignal detected by the interrogator unit.

A filter may 58 may be used to reduce or prevent pump light fromreaching the interrogator unit 14′ and affecting its performance. Thepump light may be delivered to the combiner through an additionalwavelength multiplexer (not shown), for example to implement a moresophisticated amplifier pump scheme.

One or more sections of the multi mode waveguide of the sensor fibre mayalso be used to implement a sensing operation, for example asillustrated in FIG. 5, in which a wavelength multiplexer/demultiplexer40 is used to link both the pump source 60 and a second interrogatorunit 14″ to the multimode waveguide of the first section of the sensorfibre, through the coupler.

Similar to the embodiments illustrated in FIGS. 1 and 3, multipleinterrogator units may be coupled to one or both of the single mode andmulti mode waveguides. As already discussed, interrogator unitsimplementing a Raman scattering sensing operation may particularlybenefit from being coupled to the multi mode waveguide.

The interrogator units, and also the pump source, may be considered asfunctional requirements which can be implemented using at least someshared components, such as a laser source, filters, circulators,gratings, spectometer and photodetector functions shared by one or moreof the interrogator units and possibly also the pump source.

In arrangements similar to those illustrated in FIGS. 4 and 5 the secondsection of sensor fibre may be provided by a single mode fibre, coupledthrough the amplifier to the single mode waveguide of the first sectionof fibre. Use of a single mode fibre in the second section may providebetter attenuation characteristics, resulting in better range and/orsensitivity for the sensing operations. This is a particularlyapplicable to an arrangement such as that of FIG. 4 in which themultimode waveguide of the first section is not used for sensing.

FIG. 6 illustrates use of the invention in a well bore 70. In thisembodiment the double clad fibre 10 is coupled to interrogator 11 andextends through a well head 72 into the well bore 70. The double cladfibre and interrogator can then be used to detect one or more physicalparameters within the well bore as a function of length along the doubleclad fibre 10, as already discussed above.

FIG. 7 illustrates a further application of double clad fibre asdiscussed above in distributed fibre optic sensing, as a download fibreto couple an interrogator to an installed multimode sensing fibre. FIG.7 again shows the well bore 70, such as an oil or gas well, capped by awell head 72. However, instead of a double clad fibre 10, a multimodeoptical fibre 74 is located within the well bore, and indeed may heavebeen installed there previously for a variety of purposes such ascommunication with down-hole tools and sensors, or distributed fibreoptic sensing operations such as distributed temperature sensing usingRaman backscattering within the fibre. Typically this multimode opticalfibre 74 may be a standard, commercial multimode optical fibre.Regardless of the original intended purpose, the multimode optical fibre74 may also be used for distributed vibration sensing using coherent orpartially coherent Rayleigh backscatter, even though the quality of thevibration signal will be poorer than can be achieved using single modefibre. Nevertheless, the multimode optical fibre 74 installed in thedown-hole environment will typically be protected from conditions thatmight other wise prevent multimode fibre from being used for distributedvibration sensing at all. Unfortunately, once the multimode fibre 74 hasemerged from the wellhead, it is often not sufficiently protected fromsuch conditions, rendering the entire down-hole section of fibre muchless suitable or unuseable for vibration sensing.

To enable the down-hole multimode fibre 74 to be used for vibrationsensing, a length of double clad fibre 76 provides a link or couplingbetween the multimode fibre 74 and an interrogator 75, which may besimilar or the same as interrogator 11 discussed above in connectionwith FIGS. 1 to 6. Typically, as shown in FIG. 7, the double clad fibre76 may be coupled to the multimode fibre 74 by a well head coupling 78shortly after the multimode fibre 74 has emerged form the well head, forexample within about five metres of the well head.

The waveguides of the double clad fibre may be used for one or more ofseveral functions, including delivering probe light and/or pump light tothe multimode fibre. The interrogator 75, may be coupled to the (or a)single mode core of the double clad fibre, and the single mode core ofthe double clad fibre may then be coupled to the core of the multimodefibre for example through the well head coupling 78, and theinterrogator 75 used to collect and analyses light backscattered withinthe multimode fibre 74 which has been transmitted on through the singlemode core of the double clad fibre 76 to the interrogator. Inparticular, this backscattered light may be used for coherent orpartially coherent Rayleigh backscatter detection of vibration as afunction of position along the multimode fibre 74, or for otherpurposes. To this end, FIG. 7 shows a first component 78 of theinterrogator 75 which is adapted to analyses backscattered lighttransmitted through the single mode core of the double clad fibre 76.

The interrogator 75 may also be coupled to the (or a) multimodewaveguide of the double clad fibre 76, for example using asplitter/combiner 80 which is shown in FIG. 7 as part of theinterrogator 75, so as to collect light backscattered within themultimode fibre 74 which has been transmitted on through the multimodecore of the double clad fibre 76 to the interrogator. In particular,this backscattered light may be used for purposes such as distributedtemperature sensing using Raman scattering, or for other purposes. Tothis end, FIG. 7 shows a second component 79 of the interrogator 75which is adapted to analyses backscattered light transmitted through themultiple mode core of the double clad fibre 76. The splitter/combiner 80may be a suitable mode and/or wavelength splitting and combiningcomponent, for example as described in connection with coupler 13discussed above.

Using the described or similar arrangements the multimode fibre 74 maycontinue to be used for a previously intended purpose such asdistributed temperature sensing through use of interrogator component79, while extending use of the preinstalled fibre to distributedvibration sensing. This may extend the useful life of a pre-installedoptical fibre and also avoid the expense of installing new, double cladfibre within the well bore 70, especially if the highest possibleperformance of distributed vibration sensing is not required.Alternatively, the arrangement may be used with a new installation ofmultimode fibre 74. The length of the double clad fibre 76 coupling themultimode fibre 74 to the interrogator 75 will typically be quite short(say, tens of metres) compared with the length of multimode fibre 74within the well bore (typically more than a thousand metres), so thatthe attenuation and dispersion specifications of the double clad fibrecan also be relaxed compared to the case where double clad fibre is alsoused in the well bore 74. This further reduces the cost of adding doubleclad fibre to the system.

The arrangements described in connection with FIG. 7 may be used inenvironments other than well bores, such as within or along buildings,bridges, pipelines and so forth.

Although specific embodiments of the invention have been described, itwill be apparent that a variety of modifications may be made withoutdeparting from the scope of the invention. For example, the sensor fibremay be made up of a number of sections of similar or dissimilar fibretypes, with connectors, intermediate sections of other fibre types andso forth, with at least one section having the two or more co-extendingwaveguides of the invention. The probe light injected into eachwaveguide could be identical or different, from the same or differentsources, as required or suitable for the chosen sensing operations. Thetwo or more sensing operations may be carried out simultaneously, orover separate of partially overlapping timeframes or duty cycles.

1. A distributed optical fibre sensing system comprising: a sensor fibrecomprising first and second waveguides extending together along thefibre, the first waveguide being a single mode waveguide and the secondwaveguide being a multi mode waveguide; and an interrogation systemcoupled to the sensor fibre and arranged to measure one or more physicalparameters as a function of position along the sensor fibre by carryingout at least first and second sensing operations to detect properties ofprobe light backscattered within the first and second waveguidesrespectively.
 2. The system of claim 1 wherein the first and secondadjacent waveguides are waveguides of a double clad optical fibre inwhich the single mode waveguide is a single mode core of the double cladfibre.
 3. The system of claim 1 wherein the first sensing operationutilizes Brillouin backscatter or Rayleigh scattering within the first,single mode waveguide to detect properties of probe light backscatteredwithin the first waveguide.
 4. The system of claim 3 wherein the firstsensing operation utilizes coherent Rayleigh backscatter within thefirst, single mode waveguide.
 5. The system of claim 4 wherein theinterrogation system measures vibration using the first sensingoperation.
 6. The system of claim 1 wherein the second sensing operationutilizes Raman scattering within the second, multimode waveguide todetect properties of probe light backscattered within the secondwaveguide.
 7. The system of claim 1 in which the interrogation system isarranged to carry out the sensing operations each using at least one ofRayleigh scattering, optical time domain reflectometry, coherent orpartially coherent optical time domain reflectometry, spontaneous orstimulated Brillouin scattering, and spontaneous or stimulated Ramanscattering, to measure said one or more physical parameters as afunction of position along the sensor fibre.
 8. The system of claim 1arranged to derive at least one said physical parameter by combiningproperties of backscattered probe light detected by two or more of thesensing operations.
 9. The system of claim 1 wherein the interrogationsystem is arranged to carry out the first and second sensing operationsutilizing light of different wavelengths to thereby avoid interferencebetween the first and second sensing operations arising from leakagebetween the first and second waveguides.
 10. The system of claim 1wherein the interrogation system is arranged to carry out multiplesensing operations using one of said first and second waveguides, saidmultiple sensing operations utilizing probe light of differentwavelength for each said sensing operation.
 11. The system of claim 1further comprising a pump light source coupled to the second waveguide,and wherein the sensor fibre includes a section remote from theinterrogation system configured as an optical amplifier such that probelight in the first waveguide is amplified by the optical amplifier. 12.The system of claim 1 further comprising a pump-signal combiner arrangedto optically couple the interrogation system to the first and secondwaveguides.
 13. The system of claim 12 wherein a double-clad fibresection of the sensor fibre is terminated at the pump-signal combiner,and the pump-signal combiner couples the first and second waveguides ofthe double clad fibre separately to at least first and secondinterrogation components of the interrogation system.
 14. The system ofclaim 13 wherein said first interrogation unit is coupled by thepump-signal combiner to the first, single mode waveguide of the doubleclad fibre and is adapted to determine Rayleigh and/or Brillouinbackscatter properties of the sensor fibre, and the second interrogationunit is coupled by the pump-signal combiner to the second, multi modewaveguide of the double clad fibre and is adapted to determine Ramanbackscatter properties of the sensor fibre,
 15. A distributed opticalfibre sensing system comprising: a sensor fibre having both single andmultimode waveguides extending along at least part of its length; a pumplight source arranged to deliver pump light into the sensor fibre; andan interrogator system coupled to the sensor fibre to inject probe lightinto the fibre, and to derive one or more physical parameters as afunction of position along the sensor fibre by detecting said probelight following backscatter within the fibre, the sensor fibrecomprising a first section proximal to the interrogator, a secondsection distal from the interrogator, and an optical amplifier sectionbetween the first and second sections, at least the first section of thesensor fibre comprising double clad optical fibre providing at least apart of said single mode waveguide and at least a part of said multimodewaveguide, the interrogator system being coupled to the single modewaveguide to detect probe light which has been backscattered within thesingle mode waveguide, and the pump light source being coupled to themulti mode waveguide, the system being arranged such that the opticalamplifier section amplifies said probe light using said pump light. 16.The system of claim 15 wherein the optical amplifier comprises anoptically active length of double clad fibre.
 17. The distributed fibresensing system of claim 15 wherein the interrogator system is alsocoupled to the multimode waveguide of the first section of the sensorfibre, to also detect probe light which has been backscattered withinthe multimode waveguide.
 18. The system of claim 17 wherein theinterrogator system uses a first sensing technique to detect probe lightbackscattered within one or more of the single mode waveguide sectionsof the sensing fibre, and a second sensing technique to detect probelight backscattered within one or more of the multimode waveguidesections of the sensor fibre.
 19. The system of claim 15 wherein atleast a part of the second section of the sensor fibre comprises asingle clad fibre coupled to the single mode core of the first sectionof the sensor fibre through the single mode core of the opticalamplifier section, such that the interrogator system can detect probelight backscattered within the single clad fibre using said probe lightinjected into said single mode waveguide.
 20. The system of claim 15wherein the interrogator system comprises one or more interrogator unitseach arranged to carry out a separate sensing operation to determinescattering properties of probe light within the sensor fibre.
 21. Adistributed fibre sensing system arranged to detect scatteringproperties along a sensor fibre, the sensor fibre comprising a doubleclad optical fibre.
 22. A method of monitoring an extended environment,comprising: positioning a sensor optical fibre through the environment,the sensor fibre having first and second co-extending waveguides, thefirst waveguide being a single mode waveguide of the sensor fibre, thesecond waveguide being a multimode waveguide of the sensor fibre;performing separate first and second sensing operations using the firstand second waveguides respectively to determine information on thebackscattering of probe light within the sensor fibre, as a function ofposition along the sensor optical fibre; and determining parametersrelating to said extended environment as a function of position alongsaid sensor optical fibre from said determined information.
 23. Themethod of claim 22 further comprising combining backscatteringinformation from said first waveguide with backscattering informationfrom said second waveguide in said step of determining parameters. 24.The method of claim 22 wherein said sensor optical fibre comprisesdouble clad fibre providing said first and second co-extendingwaveguides. 25-31. (canceled)
 32. The method of claim 24 wherein thesingle mode waveguide is a single mode core of the double clad fibre.33. The method of claim 22 wherein the first sensing operation utilizesBrillouin backscatter or Rayleigh scattering within the first, singlemode waveguide to detect properties of probe light backscattered withinthe first waveguide.
 34. The method of claim 33 wherein the firstsensing operation utilizes coherent Rayleigh backscatter within thefirst, single mode waveguide.
 35. The method of claim 34 wherein theinterrogation system measures vibration using the first sensingoperation.
 36. The system of claim 22 wherein the second sensingoperation utilizes Raman scattering within the second, multimodewaveguide to detect properties of probe light backscattered within thesecond waveguide.
 37. The method of claim 22 in which the interrogationsystem is arranged to carry out the sensing operations each using atleast one of Rayleigh scattering, optical time domain reflectometry,coherent or partially coherent optical time domain reflectometry,spontaneous or stimulated Brillouin scattering, and spontaneous orstimulated Raman scattering, to measure said one or more physicalparameters as a function of position along the sensor fibre.
 38. Themethod of claim 22 wherein the interrogation system is arranged to carryout the first and second sensing operations utilizing light of differentwavelengths to thereby avoid interference between the first and secondsensing operations arising from leakage between the first and secondwaveguides.
 39. The method of claim 22 wherein the interrogation systemis arranged to carry out multiple sensing operations using one of saidfirst and second waveguides, said multiple sensing operations utilizingprobe light of different wavelength for each said sensing operation.